Many applications for precise measurement of force, weight, and relative position are known in the art. For example, machine shop tools for precisely indicating or fabricating holes, channels or other surface features relative to one another require accurate position or displacement measurement. Accurate measurement of displacement or position on small parts, such as those used in the manufacture of electronic components is particularly important.
Measurement of force or weight accurately at minute quantities, along with instruments to accomplish such measurements are well known. Strain gauge transducers are one industry recognized instrument for such measurements. These instruments can be used in laboratory analysis, such as micro hardness testing of samples. Furthermore, laboratory scales for measuring constituent components in minute quantities with high resolution are well known in chemical, biological, drug and medical fields.
A known limitation to resolution in strain gauge transducers is the signal to noise ratio of the instrument. Strain gauge transducers have an output of only a few millivolts. It is recognized that the minimal possible noise level for the strain gauge transducer is set by the thermal noise on the strain gauge resistive element. For example, the calculated noise for a commercial strain gauge sensor with 350 Ohm resistance is 2.4 nV at 1 Hz bandwidth.
In more recent years, the development of scanned-probe microscopes has created a need for higher resolution measurement of force and position at minute levels. As disclosed by Wickramasinghe in "Scanned-Probe Microscopes", Scientific American, October, 1989, pp. 98-105, scanned-probe microscopes allow an examination of a surface at very close range with a probe that may be just a single atom across, and resolve features and properties on a scale that eludes other microscopes.
The disclosure of Wickramasinghe, which is incorporated herein by reference, discloses two types of scanned-probe microscopes. The first type is a scanning tunneling microscope, while the second is an atomic force microscope.
In the atomic force microscope, a scanned-probe device moves a minute tip, such as an atomically sharp diamond mounted on a metal foil over a specimen in a raster pattern. The instrument records contours of force, the repulsion generated by the overlap of the electron cloud at the tip with the electron clouds of surface atoms. In effect, the tip, like the stylus of a phonograph, reads the surface. The foil acts as a spring, keeping the tip pressed against the surface as it is jostled up and down by the atomic topography.
A scanning tunneling microscope senses atomic-scale topography by means of electrons that tunnel across the gap between a probe and the surface. Piezoelectric ceramics, which change size slightly in response to changes in applied voltage, maneuver the tungsten probe of a scanning tunneling microscope in three dimensions. A voltage is applied to the tip, and is moved toward the surface, which must be conducting or semiconducting, until a tunneling current starts to flow. The tip is then scanned back and forth in a raster pattern. The tunneling current tends to vary with the topography. A feedback mechanism responds by moving the tip up and down, following the surface relief. The tip's movements are translated into an image of the surface.
With scanning tunneling microscopy, it is recognized that measurement of surface topography would be incorrect if the tip distance from the surface is not maintained. Thus, a measurement of the force applied by the tip on the sample throughout the measurement cycle would serve to confirm that such distance is maintained, and provide a cross-check for the accuracy of the topographic measurement.
As previously stated, instruments such as strain gauge transducers can be used for micro hardness testing of samples while scanning tunneling microscopes and atomic force microscopes are recognized methods for measuring or imaging surface topography. There would be a significant advantage when making microindentation hardness tests if it were possible to immediately image the results with high resolution capability. Presently known tips and control mechanisms for scanning tunneling microscopes and atomic force microscopes have heretofore prevented these instruments from being capable of both measuring surface topography and conducting microindentation hardness tests.
The tungsten scanning tunneling microscope tips generally used on these instruments are very slender and tend to bend into a fish hook shape at rather low indentation loads so that imaging after indentation is somewhat suspect. The atomic force microscope tips, although harder than the tungsten scanning tunneling microscope tips, are mounted on a delicate cantilever which is easily broken off. This limits the amount of force that can be applied with the atomic force microscope to much less than is needed for most indentations.
An alternative approach is to build a scanning tunneling or atomic force microscope with a built in scanning electron microscope which gives the imaging capability after indentation but at a considerable expense in equipment cost and added time. Also, the scanning electron microscope only works under vacuum so that observation of moist samples, such as biological specimens is not possible.
In studying mechanical properties of materials on the microscopic scale, indentation and scratch testing are two frequently used techniques. Indentation testing, where a diamond tip is forced into the material being tested is commonly used for determining hardness, and is beginning to be used to determine elastic modulus. The scratch test is used to determine (among other things) the adhesion of a film or coating deposited on a substrate. This is done by dragging the diamond tip across the sample surface under increasing load until a critical load is reached at which time some kind of delamination or failure occurs.
Normally the indentation or scratch is performed on one machine designed for that purpose, and the results are analyzed by using a microscope to determine the indent size or area of delamination. For feature sizes of a few micrometers or greater this is usually done with an optical microscope.
For features of less than a few micrometers, as are becoming increasingly important with the continued miniaturization of semiconductors and decreased thickness of protective coatings, such as used on magnetic storage disks, the area would normally be determined by scanning electron microscope imaging. This involves significant work in sample preparation, especially for samples that are electrical insulators and need to be gold or carbon coated before imaging on the scanning electron microscope. Also, just finding the tiny indent or scratch is not trivial. For the smallest indents and scratches, the atomic level resolution of the scanning tunneling microscope or atomic force microscope may be required to accurately resolve the scratch widths and areas of delamination. Researchers have reported spending up to eight hours locating an indent on the atomic force microscope after producing it on a separate microindentor.
Another source of uncertainty is plastic flow or relaxation that may take place with certain samples. If this occurs over time periods of an hour or less, an indent produced by a separate indentor may disappear before it can be inspected on a microscope. Indents made in the 50 Angstrom range, have sometimes indicated plastic deformations that could not be seen with the scanning electron microscope or atomic force microscope imaging. Possible explanations include mechanical hysterisis in the indentor causing it to indicate plastic deformation that was not actually present. It is also possible that there actually was an indent present that the researcher was not able to locate. A third possibility is that the sample exhibited a relaxation effect where the indent was actually present, but disappeared by some plastic flow phenomena before the sample could be observed in the microscope.
There would obviously be a significant advantage when making microindentation hardness and scratch tests if it were possible to immediately image the results with high resolution capability. Such capability would both reduce time and cost of the measurements and reduce uncertainties about the results.
The process of forming an indentation in a sample for micro-mechanical testing is also limited. Forces can be applied to the sample by driving the tip into the sample material using the Z-axis piezo of a scanning tunneling microscope. This process can be controlled by writing "lithography scripts" that run under the microscope control system. These scripts can be used to control the tip motion in all three axis. Simultaneous motion in Z and X or Y directions is not supported, so the force ramp desired for continuous micro-scratch testing has to be approximated using a staircase type ramp.
The magnitude of the force which can be applied is rather limited, since it is determined by the Z-axis travel of the piezo and the spring constant of the force sensor. Higher forces could be achieved by using a sensor with a higher spring constant, but that would decrease the resolution and increase the required minimum imaging force, which may cause sample wear problems during imaging. Additionally, the Z-axis travel of the piezo actuator is not compensated for linearity and hysterisis effects, as are the X and Y axis. This results in calibration problems, since there are rather large differences between the commanded Z-axis travel in the lithography script and the actual travel of the tip in the Z-axis direction.
It would be very advantageous in micro-mechanical testing to have a mechanism which provides controlled indentation of sample material at a range extending to higher maximum forces, while maintaining a high resolution and linearity between the commanded Z-axis travel and the actual travel of the tip.
Bonin et al. (U.S. Pat. No. 4,694,687) discloses a vehicle performance analyzer which incorporates a capacitive accelerometer for detecting changes in G-forces and for producing a digital count value proportional to such changes. The sensor includes a capacitive transducer comprising a pair of spaced-apart parallel plates disposed on opposite sides of a beam-supported moveable plate, which responds to changes in acceleration of forces. Bonin et al. discloses, in FIG. 3, that the beam-supported moveable plate is sealed from access between the spaced-apart parallel plates. Thus, although not physically accessible, the moveable plate will yield and be displaced when subjected to G-forces during acceleration when mounted perpendicular to such force. Bonin et al. (U.S. Pat. No. 4,694,687) is hereby incorporated by reference.